Demand for efficient and reliable electrical power is escalating and outpacing the improvements in conventional power sources. Applications where compact, lightweight, energy-dense power supplies would find immediate application include portable power generators, combined heat and power systems, and auxiliary power units for vehicles. Concerns about global climate change are increasing with the level of CO2 in our atmosphere, caused by the use of combustion-based methods to generate power from fossil fuels. Fuel cells offer a viable approach to increase efficiency of power generation from fossil fuels, while greatly reducing emissions of pollutants and greenhouse gases. Of the various types of fuel cells, the proton exchange membrane (PEM) fuel cell, which operates with hydrogen as a fuel, is receiving considerable attention due to its low weight, low-temperature operation, and ease of manufacture. However, the operation of PEM fuel cells with fossil-based hydrocarbon fuels requires extensive pre-processing (reforming) to convert the hydrocarbons into a hydrogen rich gas, and subsequent gas purification steps to reduce carbon monoxide and sulfur to very low levels (CO<10 ppm and H2S<10 ppb). Solid oxide fuel cells (SOFCs), which operate at high temperature (typically, 600 to 1000° C.), are much less sensitive to impurities in hydrocarbon fuels, which minimizes the amount of gas purification steps required. This greatly increases power generation efficiency and reduces system complexity. It also is theoretically possible to operate solid oxide fuel cells directly on certain hydrocarbon fuels (e.g., methane, methanol and ethanol) via internal reforming, i.e., without an initial reforming step.
A solid oxide fuel cell is comprised of an oxygen ion conducting ceramic electrolyte membrane that is sandwiched by a fuel electrode (anode) and an air electrode (cathode). Power is generated by passing air (or oxygen) over the cathode and fuel (e.g., hydrogen plus carbon monoxide) over the anode, and collecting the electrical current that is created by the electrochemical reaction of oxygen with fuel to form steam and carbon dioxide. Ceramic electrolyte materials used in solid oxide fuel cells can include yttrium-stabilized zirconia (YSZ), scandia-stabilized zirconia (ScSZ), zirconium oxide doped with any combination of rare earth and/or alkaline earth elements, samarium-doped ceria (SDC), gadolinium-doped ceria (GDC), cerium oxide (ceria) doped with any combination of rare earth and/or alkaline earth elements, lanthanum strontium magnesium gallium oxide (LSGM), and other oxygen-ion-conducting ceramic electrolyte materials known to those skilled in the art. A key to successful development of SOFC systems is the electrochemical cell design and “stacking” configuration (or the manner in which SOFC elements are configured within a power-producing SOFC module). In this regard, it is important to pack a large amount of active area for electrochemical reactions within the smallest volume possible. A typical solid oxide fuel cell will generate about 30 to 40 watts of power for every 100 cm2 of active area—this translates to more than about 3000 cm2 of active area for a kilowatt of power.
Planar SOFCs, which may be supported by either the anode material or the electrolyte material, also have been demonstrated extensively. Two recent U.S. patent application Ser. No. 11/109,471 (published Oct. 19, 2006) and Ser. No. 11/220,361 (published Mar. 8, 2007), both disclosures of which are incorporated by reference herein, describe novel planar cell designs that overcome technical barriers associated with building SOFC stacks with conventional planar cells. These cell designs will generally be referred to herein as the FlexCell and the HybridCell. The FlexCell design is based on a thin electrolyte membrane layer that is mechanically supported by a “honeycomb” mesh layer of electrolyte material. With the FlexCell, more than 75 percent of the electrolyte membrane within the active area is thin (20 to 40 microns), and the periphery of the cell is dense. Electrode (anode and cathode) layers are subsequently deposited onto the major faces within the active cell regions to complete the fabrication of an SOFC based on the FlexCell design. With the HybridCell, an anode layer (30 to 40 microns) is co-sintered between the mesh support layer and the electrolyte membrane layer. With the HybridCell, the entire active cell area has a thin electrolyte membrane (10 to 20 microns), and the periphery of the cell is dense. The cathode layers are subsequently deposited onto the major faces within the active cell region to complete the fabrication of an SOFC based on the HybridCell design.
In order to generate useful amounts of electrical power, planar SOFCs are configured in a “stack”, with multiple planar cells separated by planar electrical interconnect components that conduct electricity between the cells and define the flow paths for oxidant (air or oxygen) through the cathode channels and fuel (H2, CO, CH4, etc.) through the anode channels (see FIG. 1). In some planar stack designs, conductive foams or meshes are included within the stack to facilitate current collection. For example, cathode current collectors are placed between the interconnect and the cathode face of the planar SOFC cell, and anode current collectors are placed between the interconnect and the anode face of the planar SOFC cell. The present invention addresses these current collector components and, in particular, anode current collector components.
For SOFCs to be of practical application, they must operate using fuels that are easily available. This requires that power supplies operate on conventional fuels, such as natural gas, propane, gasoline, and diesel. Typically, a hydrocarbon fuel is pre-reacted (reformed) over a catalyst with air and/or steam to produce a mixture of H2 and CO (and in some cases CH4) gas before delivery to the fuel cell. Promising development is underway to provide compact and lightweight reformers for conventional fuels. However, traditional fuels contain some level of sulfur. Sulfur can have devastating effects on conventional SOFC performance. Cermet mixtures, which comprise both a metallic component and a ceramic component, and, in particular, typically comprise mixtures of nickel metal with electrolyte materials (YSZ or GDC), are the most common SOFC anodes, but are susceptible to sulfur poisoning in concentrations as low as a few ppm. This leads to significant performance degradation, especially at lower operating temperatures (700 to 800° C.) which are desired for SOFC stacks that utilize inexpensive metallic interconnect components. A recent patent application, U.S. patent application Ser. No. 12/001,062, filed on Dec. 7, 2007, the disclosure of which is incorporated by reference herein, discloses SOFC anode compositions that are tolerant to sulfur and comprise a composite of a nickel-cobalt alloy and a ceria-based electrolyte material (e.g., samarium doped cerium oxide).
With this multilayer nickel-cobalt/ceria anode formulation, sulfur tolerance has been demonstrated when hydrogen is the fuel (see FIG. 2). Without intending to be bound by any theory, it is believed that the mechanism for this sulfur tolerance involves the conduction of both oxygen ions and electrons through the anode, which results in the conversion of H2S to SOx, thereby minimizing the absorption of H2S onto active sites (see FIG. 3). Although this mechanism of sulfur tolerance provides exceptional sulfur tolerance when hydrogen is the fuel, sulfur is rarely present in pure hydrogen. Sulfur is present in fuels that are derived from hydrocarbon fuels, so that after reforming the fuel consists essentially of hydrogen and carbon monoxide in conjunction, with residual H2S. For optimum efficiency of power generation in an SOFC, this carbon monoxide must be utilized as fuel. When sulfur is not present, the carbon monoxide is consumed by the sequential steps of: (1) water gas-shift reaction, whereby CO and H2O react to form CO2 and H2; and (2) electrochemical oxidation of H2 to H2O at the SOFC anode. In essence, the water-gas-shift reaction followed by electrochemical oxidation of hydrogen is much faster than direct electrochemical oxidation of CO. Unfortunately, the presence of H2S poisons the water-gas-shift reaction, so that CO cannot be efficiently utilized as fuel in an SOFC. The sulfur tolerant anodes have provided improved SOFC performance.
However, there remains a need for SOFC's with further improved performance, particularly using readily available hydrocarbon fuels.